Disintegration of vegetable fibrous materials



Patented Jan. 9, 1940 UNITED STATES PATENT OFFICE DISINTEGRATION FVEGETABLE FIBRO'US MATERIALS No Drawing. Application May '1, 1931,Serial No.

141,251. In Germany May 13, 1936 Claims.

My present invention relates to an improvement in a process fordisintegrating wood and other cellulose-containing material and moreparticularly to such a process wherein nitric acid is used fordecomposing the lignin and the other compounds accompanying thecellulose.

In my copending application Ser. No. 62,815, filed February 7, 1936,there is described a process for disintegrating wood in which the woodis placed in a pressure vessel and covered therein with a weak nitricacid of at least 4 per cent. strength, the initial temperature being atmost 60 C. and the air being pumped from the vessel or not as desired.In the closed vessel in which the acid is continuously circulatedbypumping, the temperature rises in the course of some hours to '70 to90 C. and the excess pressure produced is maintained at severalatmospheres with occasional blowing ofi, until the reaction is complete.

is drawn off and usual after-treatment with alkali and washing follow.The process is characterized by its small consumption'ot nitric acidwhich lends it commercial efllciency.

The present invention reduces still) further the consumption of nitricacid and in consequence of the use of larger pieces of wood with lesswaste splinter a larger yield of cellulose is obtained The inventionconsists in a two-stage process and is applicable to various vegetablefibers, for instance, wood, straw, grasses or the like; it will here bedescribed as applied towood. The wood in the form of shavings isintroduced into an-acid resisting pressure vessel which is evacuated ofair; the wood is covered with a solution of nitric acid of 10 to 15 percent.

strength by volume, the amount being in excess of that necessary todisintegrate the wood. Whe

the whole of the nitric' acid has been introduce there is produced inthe vessel an excess pres sure of about 5 atmospheres by introducingcompressed air or by a pressure development during the reaction afterthe vacuum has been broken. The acid soaks energetically into the wood.At first by suitable attemperating the acid the temperature in thevessel ismaintained at about 30 to about 40 C., but in the course of afew hours the temperature is allowed to rise gradually to about 40to'about 50 C. respectively, the heat of reaction being used for thispurpose; the pressure is kept the same as in the first phase by suitableblowing off the carbon dioxide which is produced. The wood is by nowsaturated with the quantity of nitric acid necessary for thedisintegration and, as indicated by the heat of the Alter this singlestage operation the nitric acid' a made of Krupps rustless steel ischarged with 100 "kilos of air-dry beech-wood shavings and the airreaction, the disintegration has already been begun. The acid which hasnot been taken up by the shavings is now removed and after being made upto strength may serve for the next disintegration; then so much hotwater or to C. is 5 introduced that the temperature in the vessel isabout 70 C. In the course of 6 'to 8 hours the temperature is raised bystages to about C. to about C. In this second phase the disintegrationproper proceeds. It a medium or high viscosity pulp is required thegases formed in this stage of the process are entirely blown oil so thatno pressure is produced in the vessel. 'When a low viscosity pulp isdesired an excess pressure is maintained also in the second phase, theamount of this being adjusted to suit the desired viscosity of the pulp.

The water feebly acid with nitric acid is now run oil and the pulp isIree'das completely as possible from the adhereing acid by washing withhot water or hot alkali lye. By the usual aftertreatment with a warm,weak caustic soda solution the oxidized lignin constituents aredissolved. The pulp is now washed free from alkali and is then in a formeasily separated into fibers and ready for further'operation 'forproducin pure cellulose.

The consumption of nitric acid owing to the mode oi application in thefirst stage of the process may be limited to the smallest quantitynecessary for the disintegration.

The followi g examples illustrate the invention: 3:?

Example 1'.--An acid resistant pressure vessel is pumped from it in thecourse of- 30 minutes until the vacuum is about.60 mm.; 300 liters of.nitric-acid solution of 13.per cent. strength by volume are then runin. When the whole of the 40 acid has been added, air is admitted tobreak the vacuum and is then pumped in until a pressure of 5 atmosphereshas been attained. The temperature of the acid introduced is so adjustedthat after the vessel has been charged the initial 45 temperaturetherein is about 35 C. In the course of 3 hours'during which the acid iscontinuously circulated by pumping, the temperature due to theexothermic reaction rises uniformly to 45 C. By permitting the carbondioxide produced to 50 escape, pressure in the vessel is kept constantat about 5 atmospheres. The wood has by now become completelysaturatedwith the quantity of nitric acid necessary for thedisintegration, so that the excess acid may be run off. liters of waterat 85' C. are then added and the temperature in the vessel is adJustedto 70 C. This temperature is raised by stages in the course oi about 7hours to 90 C., if necessary with the addition of heat, the gases beingfree to escape sothat the pressure does not rise and the disintegrationproper of the wood is can-led through without pressure.

The feebly acid water (containing about 0.7 per cent. by volume of HNOa)is now run out and the wood is freed from adhering acid as far aspossible by washing with hot water or hot waste alkali lye. Therefollows a usual after-treatment with a caustic soda lye of 3 per cent.strength by volume continuing for about 2 hours at 80 C.; in this mannerthe oxidized lignin constituents are dissolved. The pulp is then washedfree from alkali, whereby it is obtained in a form easily separated intofibers very suitable for being further worked into pure highly viscouscellulose.

For obtaining a cellulose of low viscosity, an excess pressure of one ormore atmospheres should be maintained after the addition of the hotwater, that is to say, in the second phase; after-treatment remains thesame.

Example 2.-l kilos of air-dry pine-wood shavings are introduced into anacid resistant pressure vessel and the air is pumped from it until thevacuum is about 60 mm. mercury. Then 400 liters of nitric acid solutionof 14 per cent. strength by volume are soaked in. When the whole of theacid has been added, the vacuum is broken by the admission of air and bypressing air into the vessel a pressure of 3 atmospheres is attained.The temperature of the acid is so adjusted that after the vessel hasbeen charged the initial temperature therein is about 40 C.

While continuously circulating the acid by pumping, the temperature isallowed to rise in the course of 3 hours due to the exothermic reactionuniformly to 50 C. By permitting the carbon dioxide formed to escape,pressure in the vessel is kept constant at about 3 atmospheres. The woodis now completely saturated with the acid and contains the quantity ofnitric acid necessary for the disintegration and the excess of acid isallowed to run oil. 185 liters of water at 85 C. are then introducedinto the vesseland the temperature in the vessel is adjusted to 70 C.This temperature is raised by stages inthe course of 8 hours to 95 C.,if necessary by heating and an over-pressure of 1 atmosphere ismaintained in the vessel until the disintegration is complete. The pulpthus obtained is freed from acid and the oxidized lignin constituentsare further worked up as indicated in Example 1.

It is obvious that my invention is not limited to the foregoing examplesor to the specific details given therein and practically the processareas tion of nitric acid of about 10 to about 15 per cent. strength byvolume, completing the impregnation by applying pressure up to aboutatmospheres, the initial temperature of about 30 C. to about 40 C. beingraised to about 40 to about 50 C. respectively, while maintaining saidpressure, romoving the nitric acid not taken up by the wood, adding hotwater so that the temperature is about 70 C. and completing thedisintegration at a temperature of about 70 to about 95 C.

2. In the process for disintegrating wood by means of nitric acids thesteps which comprise impregnating the wood under vacuum with a solutionof nitric acid of about to about per cent. strength by volume,completing the impregnation by applying pressure up to about 5atmospheres, the initial temperature of about to about C. being raisedto about 40 to about C. respectively, while maintaining said pressure,removing the nitric acid not taken up by the wood, adding hot water sothat the temperature is about C. and completing the disintegration at atemperature of about 70 to about C. at atmospheric pressure.

3. In the process for disintegrating wood by means of nitric acids thesteps which comprise impregnating the wood under vacuum with a solutionof nitric acid of about 10 to 15 per cent. strength by volume,completing the impregnation by applying pressure up to about 5atmospheres, the initial temperature of about 30 to about 40 C. beingraised to about 40 to about 50 C. respectively, while maintaining saidpressure, removing the nitric acid not taken up by the wood. adding hotwater so that the temperature is about 70 C. and completing thedisintegration at a temperature of about 70 to about 95 C. and under apressure of up to about 5 atmospheres.

4. The process for disintegrating beech-wood shavings which comprisesetting about kg. of the air-dry shavings under vacuum, adding 300liters of nitric acid of 13 per cent. strength, breaking the vacuum byair, providing a pressure of 5 atmospheres, the initial temperaturebeing 35 C., circulating the acid and raising the temperature to about45 C., keeping the pressure at about 5 atmospheres, removing the acidnot taken up by the wood, adding liters of water of 85 C., raising thetemperature in the course of about 7 hours to 90 C. and completing thedisintegration at atmosphericpressure.

. '5. The process for disintegrating pine-wood shavings which comprisessetting about 100 kg. of the air-dry shavings under vacuum, adding 400liters of nitric acid of 14 per cent. strength, breaking the vacuum byair, providing a pressure of 3 atmospheres, the initial temperaturebeing 40 C., circulating the acid and raising the temperature to about50 C., keeping the pressure at about 3 atmospheres, removing the acidnot taken up by the wood, adding liters of water of 85 C., raising thetemperature in the course of about 8 hours to 95 C. and complet ing thedisintegration at a pressure of one atmosphere gauge.

ALFONS BAYERL.

